<!DOCTYPE html>
<html lang="en">

<head>
  <!-- initial-scale初始缩放 -->
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=2.0">
  <title>Document</title>
  <style>
    div {
      width: 200px;
      height: 200px;
      background-color: orange;
    }

    /* 适配手机  屏幕小于750px */
    @media screen and (max-width:750px) {
      div {
        width: 150px;
        height: 150px;
        background-color: green;
      }
    }

    /* pad  大于750px 小于1200px */
    @media (min-width:750px) and(max-width:1200px) {
      div {
        width: 200px;
        height: 200px;
        background-color: yellow;
      }
    }

    /* 大于1200px */
    @media screen and(min-width:1200px) {
      div {
        width: 500px;
        height: 500px;
        background-color: pink;
      }

      body {
        background-color: aqua;
      }

    }
  </style>
</head>

<body>
  <div></div>
</body>

</html>